The present invention relates to the roto-molding of plastic containers and more specifically to a method and apparatus for roto-molding wherein a message or other indicia can be molded directly into the outer surface of the plastic container.
Roto-molding is a well known process for manufacturing plastic containers. Briefly, in roto-molding a charge of powdered thermoforming resin, such as polyethylene, polypropylene or the like is placed into a hollow mold. The mold is then heated to temperatures which may range up to 700.degree. F. depending upon the resin material. The mold is simultaneously rotated about two axes while it is being heated. This distributes the powdered resin over the entire interior surface of the mold and fuses the resin into a seamless homogeneous structure with a uniform wall thickness. The mold is then cooled, opened and the plastic container stripped from the interior of the mold.
Heretofore, when it was desired to form any indicia, such as liquid level lines, company names, or other appropriate messages into the side of the roto-molded plastic container, it was necessary to physically engrave a mirror image of the indicia into the wall of the mold. Thus, considerable time, effort and expense was required to change the mold in this fashion and consequently, the roto-molding of containers with indicia or other markings molded directly into the exterior of the container is seldom, if ever, undertaken.
The use of removable templates temporarily fixed to the interior surface of the roto-mold likewise has not heretofore proven successful. This is due in large measure to the problem of preventing the template from becoming detached from the side wall of the mold upon exposure for relatively long durations to the high temperatures involved in roto-molding, sometimes approaching 700.degree. F. for periods of 10-15 minutes or more. Further, the air present in the space between the template and the side wall of the mold during the heating cycle expands and causes defects in the surface of the molded article on cooling.
The present invention, however, does provide a method and apparatus for modifying a roto-mold so as to permit the molding of indicia into the side of the roto-molded plastic container. In this respect, the invention involves in one aspect, the use of a metal template which has engraved on its surface a mirror image of the indicia to be molded into the side of the plastic container. This metal template is mechanically fastened to the interior surface of the roto-mold by screws, bolts or the like with a RTV (Room Temperature Vulcanizing) silicone rubber used as a caulking material to prevent the formation of bubbles in the plastic container due to air entering from around the template.
In another embodiment, the template itself, is cast in a silicone RTV rubber, the template then being attached to the mold using additional RTV silicone rubber as the adhesive agent. This latter method is preferred in that it does not destroy the integrity of the mold as compared to the mechanical technique defined above.
It is therefore the primary object of the invention to provide a method and apparatus for roto-molding messages into the outer surface of a plastic container without the need for engraving the indicia into the wall of the mold.
The invention will be better understood and additional objects thereof will become more apparent from the accompanying specification and claims considered together with the accompanying drawing.